Digital imaging systems such as digital cameras continue to increase in popularity, providing users with the ability to capture images (i.e., take photographs) with relative ease. Digital imaging systems typically include a lens for directing the light comprising a digital image through a light path to an optical sensor array. Autofocus systems (as well as other automations such as automatic exposure or flash) are often an important part of digital imaging systems as they improve the user experience by making such systems easier to use. Whether an object in an image is ‘in focus’ (i.e., at the sharpest possible setting) for a digital imaging system depends on a number of factors, including the distance between the lens and the sensor array, the lens focal length, exposure aperture, and the distance to the subject. As subject distance can effect whether an object is in focus, some objects in an image may be ‘in focus’ while other objects may be ‘out of focus’. Autofocus systems typically include a focusing motor that moves a portion of the lens of the digital imaging system in and out until the sharpest possible image of the desired subject is projected onto the optical sensor array. In manual focus systems, a user would turn a focusing ring on the lens until the image (or portion of an image) in the viewfinder appeared in focus.
Autofocus systems typically rely on active autofocus, passive autofocus, or a combination of the two, and utilize one or more autofocus sensors within the field of view. Active autofocus systems measure the distance to the subject (using, for example, sound or infrared signals) and adjust focus of the optical system accordingly. Passive systems analyze the incoming image itself and drive the lens back and forth searching for the best focus and can include both phase detection systems and contrast measurement systems. Complicated autofocus systems with many sensors can add significant cost and complexity to a digital imaging system, as autofocus sensors are relatively expensive and more accurate sensors (e.g., horizontal and vertical capability) are more expensive still. In all but the most expensive digital single-lens-reflex (DSLR) camera, there will typically be only a few autofocus sensors within the user's field of view. Because autofocus sensors do not completely cover the field of view, the subject that the user desires to be in focus may not lie beneath an autofocus sensor, making it difficult to focus on the subject. In this case, users typically rotate the camera until an autofocus point (typically the center autofocus sensor, as it is usually the most accurate) falls over the area of interest and then lock the focus. After locking the focus, the user then may recompose with the subject at the desired location in the frame and then take the exposure.
The solution of locking the autofocus and recomposing often provides unacceptable results, however, when the depth-of-focus (DOF) is small compared to the difference in subject distance between the scene composed as desired as the scene composed during focus lock. A user taking a portrait (where a very small DOF created by large apertures is often aesthetically desirable), for example, might lock focus on the subject's eyes but when the user recomposes, the plane of focus would be behind the eyes. Thus, if the DOF is too small the subject's eyes become out-of-focus and an undesirable photograph results. The DOF may vary depending on imaging sensor size, imaging lens focal length, exposure aperture, and subject distance. For close subject distances and/or large exposure apertures with small DOF, the problem is exacerbated and unacceptable focus shifts are introduced. There is, therefore, a need for an effective system to provide improved autofocus for digital imaging systems.